The detection of motions of a motor vehicle is considered particularly significant in the automobile industry. Motion variables such as, for example, the longitudinal speed of a vehicle, constitute important information for the driver of the vehicle. In addition, motion variables serve as input signals for vehicle movement dynamics control systems with which the safety and comfort of the vehicle are improved. A conventional vehicle movement dynamics control system generally comprises in this context wheel speed sensors, from whose measured values the longitudinal speed of the vehicle is derived, and a sensor for detecting the yaw rate of the vehicle, and sensors for detecting the transverse acceleration and, if appropriate, the longitudinal acceleration.
DE 10 2004 060 677 A1, which is incorporated by reference, discloses an optical device for obtaining the speed of a motor vehicle, in which device images of the underlying surface of the vehicle are recorded in a sequence. The speed of the vehicle is obtained on the basis of displacement of objects or structural features within the images. In addition to the longitudinal speed, it is also possible to obtain the transverse speed of the motor vehicle in this context, the transverse speed frequently being inaccessible to measurement by means of the vehicle sensor system and constituting a useful input variable for vehicle movement dynamics control systems.
Such a device generally requires additional lighting of the roadway surface, as a result of which the expenditure on integration into a motor vehicle is increased. In addition, the device is relatively susceptible to soiling of the camera or of the lighting device, which occurs to a greater degree owing to the installation position on the underfloor of the motor vehicle. As a result, the availability of the device can be adversely affected. Furthermore, the device can only detect speed components in the plane of the roadway but not perpendicularly thereto.